The present invention relates, in general, to an improved spray gun for producing charged fluid particle sprays, and more particularly to induction charging apparatus for a high volume, low pressure fluid spray device and to a method for inducing charges on conductive atomized fluids.
This invention is related to that disclosed in U.S. Pat. No. 5,044,564 issued to James E. Sickles on Sep. 3, 1991, the disclosure of which is incorporated herein by reference.
Conventional airless, air assisted, or air atomization spray guns incorporate a spray cap having a spray nozzle, the nozzle portion of the cap including liquid passageways and some mechanism for atomizing a liquid such as paint. In such devices, the liquid flows under pressure or is siphoned through a central passageway in the cap for discharge through a central outlet orifice. This liquid flow is typically controlled by a flow control needle valve located in the central passageway, and the size of the orifice and the pressure of the operator's hand on the spray gun trigger is selected so that the liquid is atomized as it is discharged. In an air assisted or air atomized spray gun, air outlets are provided near the central liquid orifice to assist in the atomization and to control the direction and flow pattern of the resulting liquid particles or droplets. Thus, air under pressure may be supplied coaxially with the liquid ejected from the liquid outlet orifice to further atomize the liquid and to impel the droplets outwardly away from the spray gun nozzle. This air flow typically is through a single annular orifice surrounding the liquid outlet, although additional air outlet orifices may be provided at locations spaced outwardly from the liquid outlet. In addition, air may be supplied by a pair of forwardly projecting air horns mounted on the spray cap, the air horns incorporating additional air outlets directed generally inwardly toward the axis of the atomized spray to control its pattern. Typically, these air horns shape the atomized spray into a fan pattern to facilitate operation of the spray gun, with the air cap being positioned on the spray gun to provide, for example, a vertical fan or a horizontal fan pattern.
The use of such conventional spray guns for spraying materials such as paint having a high solids content creates problems, since such spray guns have low transfer efficiencies, in the range of 15 to 30% for an air-atomized paint spray. Increased efficiency has been obtained through electrostatic charging of the atomized coating material, such charging increasing the efficiency to the range of 45 to 75% for electrostatic air atomized spray devices and from 90 to 99% for electrostatic rotary bell spray devices. However, even electrostatic devices present problems, particularly when spraying a conductive liquid such as water-based paint, for it is necessary to electrically isolate such a system to prevent high voltages from endangering users or causing electrical discharges which could result in fires or explosions. Various techniques have been provided for producing the necessary isolation, but difficulties have been encountered in each such system.
Most prior electrostatic air spray or air-assisted spray devices have in common a spray gun to which is mounted a high voltage electrode disposed adjacent the spray discharge point or more commonly, in direct contact with the liquid stream itself, and carrying an electrical potential in the neighborhood of 50 to 85 KV, and in some instances as high as 150 KV. Such a device is illustrated, for example, in U.S. Pat. No. 4,761,299, where a voltage on the order of 100 KV is applied between the spray gun electrode and the article being sprayed. In addition to providing high voltage contact (or conduction) charging of the spray droplets by direct physical contact of the liquid with the electrodes, the electric field produced by this voltage creates a field rich in gaseous ions through which the spray particles must pass so that some of the ions become attached to the particles. This produces electric charges on the particles of the same polarity as that of the high voltage electrode, causing them, together with copious quantities of free, unattached ions, to migrate toward the grounded workpiece. It has been found that the free ion current deposited on a grounded target can be up to several times that deposited by charged spray particles.
Such electrostatic, or corona effect, devices encounter numerous difficulties, not only because of the very high voltages required to produce effective operation, but because a significant part of the current between the spray gun and the target, or workpiece, is due to free ions, rather than charged particles, thereby reducing transfer efficiency. The high voltages are a problem because they require large, heavy and relatively expensive power supplies and because the cable interconnecting the power supply and the spray gun charging electrode necessarily has to be heavily insulated, making it bulky, relatively inflexible, and expensive. The size and weight of the power supply and its cable substantially restricts the usefulness of conventional corona effect spray guns.
Various attempts have been made to overcome the power supply problem of such high voltage devices, but with limited success. The use of high voltages, furthermore, is hazardous not only because of the possibility of creating electrical arcing when the gun is moved near grounded objects, but because of the danger to the operator if the electrode is inadvertently touched. Furthermore, the high voltages used in such systems create a current flow of excess ions travelling to nearby objects, in addition to the target, resulting in an undesired charge build-up on such nearby objects if they are not adequately grounded. The hazard of sparking and consequent fire exists when the operator or some other grounded object is brought close to such a charged object. Further, the migration of such charges causes an undesired build-up of the charged spray particles on objects other than the workpiece.